Investigating CN^– cleavage by three-coordinate M[N(R)Ar]₃ complexes

Three-coordinate Mo[N(^tBu)Ar]₃ binds cyanide to form the intermediate [Ar(^tBu)N]₃Mo–CN–Mo[N(^tBu)Ar]₃ but, unlike its N₂ analogue which spontaneously cleaves dinitrogen, the C–N bond remains intact. DFT calculations on the model [NH₂]₃Mo/CN^– system show that while the overall reaction is significantly exothermic, the final cleavage step is endothermic by at least 90 kJ mol^–1, accounting for why C–N bond cleavage is not observed experimentally. The situation is improved for the [H₂N]₃W/CN^– system where the intermediate and products are closer in energy but not enough for CN^– cleavage to be facile at room temperature. Additional calculations were undertaken on the mixed-metal [H₂N]₃Re⁺/CN^–/W[NH₂]₃ and [H₂N]₃Re⁺/CN^–/Ta[NH₂]₃^– systems in which the metals ions were chosen to maximise the stability of the products on the basis of an earlier bond energy study. Although the reaction energetics for the [H₂N]₃Re⁺/CN^–/W[NH₂]₃ system are more favourable than those for the [H₂N]₃W/CN^– system, the final C–N cleavage step is still endothermic by 32 kJ mol^–1 when symmetry constraints are relaxed. The resistance of these systems to C–N cleavage was examined by a bond decomposition analysis of [H₂N]M–L₁L₂–M[NH₂]₃ intermediates for L₁L₂ = N₂, CO and CN^– which showed that backbonding from the metal into the L₁L₂ π* orbitals is significantly less for CN^– than for N₂ or CO due to the negative charge on CN^– which results in a large energy gap between the metal d_π and the π* orbitals of CN^–. This, combined with the very strong M–CN^– σ interaction which stabilises the CN^– intermediate, makes C–N bond cleavage in these systems unfavourable even though the CN triple bond is not as strong as the bond in N₂ or CO.